For example, in a projector for image display such as a liquid crystal projector or a DLP™ projector, a high intensity discharge lamp (HID lamp) is used. In some types of such projectors, light is separated into three primary colors of red (R), green (G), and blue (B) by a dichroic prism etc., so that a space modulation element provided for each color generates an image of each of the three primary colors, and the optical paths thereof are combined by a dichroic prism etc., thereby displaying a color image. In another known type of projector, a filter having three primary color areas (R, G, and B) is rotated, and light emitted from a light source is passed through this filter, thereby sequentially generating light rays of the three primary colors, and then the spatial modulation elements are controlled in synchronization with the generated light rays so as to sequentially generate an image of each of the three primary colors in a time dividing manner, thereby displaying a color image.
The high intensity discharge lamp used as a light source for such a projector may be, for example, a high pressure mercury lamp, a metal halide lamp, or a xenon discharge lamp etc. However, a xenon discharge lamp, in which the emission spectrum thereof is similar to sunlight and large electric power can be comparatively easily realized, is used in a dedicated manner for a high-definition image, to which a greater importance is attached to the color reproduction nature, or for a large area screen image in a movie theater etc.
An example of a schematic structure of such a xenon discharge lamp is shown in FIG. 12. In an outer enclosure (10), which surrounds an electrical discharge space (Es) containing xenon gas as a main component, and which is made from a heat-resistant and high transparent material such as silica glass etc., a cathode electrode (E1) and an anode electrode (E2), which are made from a heat-resistant conductive material such as tungsten etc., are arranged so as to face each other, wherein arc discharge is generated between both electrodes. During lighting, electrons, which are emitted from the cathode electrode (E1) and reach the anode electrode (E2) release kinetic energy as heat, so that the anode electrode (E2) generates heat remarkably. Therefore, the thickness and length of the anode electrode (E2) need to be large comparing with those of the cathode electrode (E1), in order to raise the heat dissipation efficiency. The cathode electrode (E1) and the anode electrode (E2) are respectively connected to cathode and anode side caps (03, 05) through metallic foils (02, 04) made of molybdenum, for supply of electricity. In the case of the lamp of FIG. 12, in order to efficiently use light emitted from the arc discharge space, a concave mirror, which has a paraboloidal face or elliptical face shape, is provided near the lamp, and the light is guided to an optical system of a following stage such as a light tunnel.
On the other hand, FIG. 13 shows an example of a schematic structure of a xenon discharge lamp, wherein an outer enclosure (70) surrounding an electrical discharge space (Es) serves as the above-described concave mirror, and such a xenon discharge lamp is proposed in Japanese Patent Application Publication No. H09-161727. A cathode electrode (E1) is connected to a cap (64) for a cathode through electric conduction supporters (61, 62, 63) made from high heat-resistant and conductive material such as a molybdenum sheet etc., and an anode electrode (E2) is directly connected to a cap (65) for an anode. Although the outer enclosure (70) is made from high heat-resistant ceramic material such as alumina etc. since no transparency is required therefor, a light extraction window (71) is made from the transparent material with high heat resistance and high mechanical strength such as sapphire etc. Metal covers (66, 67) for airtightness and protection are provided on connecting faces of the outer enclosure (70) between the cap (64) for a cathode and the cap for (65) an anode. Electric supply connections with the cap (64) for a cathode and the cap (65) for an anode are made through a conductive radiation fins.
In a discharge lamp lighting apparatus for lighting the above-mentioned xenon discharge lamp, first, high voltage is impressed to the lamp by a starter in a state where voltage called no-load open voltage is impressed to the lamp when it is started, so that dielectric breakdown is generated in the electrical discharge space, and inrush current having a suitable peak value is supplied thereto, in order that it shifts to arc discharge for starting, whereby an operation is finally performed so that stable steady lighting may be realized. Usually, such a discharge lamp lighting apparatus has a converter which adjusts an output of an input power supply for lamp discharge voltage so that target lamp current, which is required in order to realize predetermined power to be applied to the lamp, can be outputted. Moreover, the lamp voltage, i.e., output voltage of the converter, may be detected, and target lamp current is determined based on this information, by, for example, a quotient value which can be obtained by dividing the target power by the detected voltage.
By the way, it is desirable that the light source lamp have a long life span in not only the projector described above, but also all uses. However, in such a xenon discharge lamp, temperature management during lighting is important for the extension of life span. Although it may seem to be advantageous with respect to extension of life span for the temperature of each part of the lamp to be low, this is not actually the case—the life span of some parts is actually shortened if their temperature is too low.
The life span in continuous lighting conditions of a xenon discharge lamp that is designed so that the temperature of the cathode electrode may become low during lighting may become long since there is suppressing effect in consumption of the cathode due to electric discharge. However, in such a case, a blackening phenomenon of a lamp bulb occurs at start-up time of the lamp, whereby there is a problem that a life span thereof becomes shorter than a life span of those which are designed so that the temperature of a cathode electrode may not become too low. This can be easily checked by performing flashing-lighting. Therefore, in the past, a lamp has been designed so as to optimize the temperature of the cathode electrode so that the life span under the operating condition of flashing-lighting may become the longest.
On the other hand, in order to make color reproduction performance of a display image high, the spectrum distribution of the light source lamp and adjustment of a conversion form to color sequential light flux using the above-mentioned dynamic color filter are important. In the case of the above-mentioned color wheel, when the angle distribution of each color of R, G, and B (in some case, additionally W, i.e. white), that is, the amount of time that each color transmits per one rotation, is set up according to the spectrum of the lamp, it is possible to improve color reproduction performance or to make improvements to obtain a desired color reproduction performance.
For example, when a B component runs short, it is effective to make an area, where the B component passes through, large, that is, to make the amount of time during which the B component passes through the filter, longer than that of other colors. However, when an improvement is made to obtain desired color reproduction performance by such a method in a DLP type projector, there is a problem that it becomes difficult to perform fine control of gradation of a pixel in a color component in which the rate of transmission time is reduced, since the luminance of each color of every pixel of a display image in a DLP type projector is controlled based on a duty cycle ratio in an operation of each pixel of a spatial modulation element. Moreover, when the color reproduction property is dynamically changed, the fine control of gradation will be impaired.
In order to solve such a problem, for example, in Japanese Patent Application Publication No. 08-505031, it is proposed that in an image projection apparatus, a light source drive control unit is provided to change the output power of a light source in synchronization with the color of the optical beam given by the output of a color change unit, whereby light source intensity modulation is performed.
Moreover, in Japanese Patent Application Publication No. 02-119005, a light device, which adjusts the light emission light intensity of a light source according to the color of a filter region in synchronization with the rotation of a rotation color wheel, is proposed. In addition, although this literature is not intended for a projector (rather, an endoscope apparatus is targeted), the above-described case does not differ from the literature in terms of performing light source intensity modulation. Therefore, the situations thereof are the same as each other with respect to a problem, which is described below and which is caused by the light source intensity modulation.
Thus, in order to solve the above-mentioned problem of increasing color reproduction performance, it is known that it is useful to perform light source intensity modulation synchronized with the conversion operation to the color sequential light flux, which uses a dynamic colored filter. However, in the case of a xenon discharge lamp, which is especially designed so that the temperature of a cathode electrode may become low, when lighting is performed by the discharge lamp lighting apparatus in which output current modulation is performed for such light source intensity modulation, there is also a problem that blackening of a lamp bulb tends to occur in addition to the above-mentioned blackening phenomenon.
For solutions to this problem, Japanese Patent Application Publication No. 2007-280822 discloses “a discharge lamp lighting apparatus (Ex) for lighting the discharge lamp (Ld) in which an electric discharge medium, which contains xenon, is enclosed in an electric discharge container, a pair of cathode electrode (E1) and an anode electrode (E2) for main discharge is arranged to face each other, and at least the cathode electrode (E1) contains electron emissive material, wherein the discharge lamp lighting apparatus (Ex) has a starter (Us) for generating dielectric breakdown in the electric discharge container of the discharge lamp (Ld) by generating high voltage at start-up time, and a power supply circuit (Ux) for supplying discharge current to the discharge lamp (Ld), wherein the power supply circuit (Ux) has an output current modulation circuit (Um) for modulating the magnitude of current passed through the discharge lamp (Ld) in at least a lighting steady state, according to a modulation signal (Sm), and wherein while the output current modulation circuit (Um) controls speed of change at which the magnitude of lamp current per square millimeter in a cross section of the cathode electrode (E1) increases, so as to 3.9 A or less per millisecond, average current in the lighting steady state is set in the power supply circuit (Ux), to a value in a range where a variant part is formed at a tip part of the cathode electrode (E1).
That is, after specifying the speed of change at which the magnitude of lamp current increases by the output current modulation, the average current in the lighting steady state is set to a value in the range where the variant part is formed in the tip part of the cathode electrode (E1). However, it turns out that when lighting, for which intense output current modulation for light source intensity modulation is performed, continues in such a discharge lamp lighting apparatus, there is electric discharge instability, which tends to occur when output current modulation is eased after the intense modulation. That is, it turns out that when the lighting state in which rectangle pulsed current is periodically superimposed to a certain level of lamp current, continues, and then superposition of the pulsed current is stopped or the magnitude of the pulsed current to be superimposed is decreased, electric discharge instability occurs sometimes.
An example of this electric discharge instability is given in a concrete application: after lamp current is increased by performing output current modulation of, for example, a specified color segment(s) of one or more colors in the color wheel, in synchronization with the timing at which a beam from a discharge lamp passes therethrough, so that an operation in which the specific color(s) is reinforced and compensated for projection in a projector image, when the increased lamp current is reduced by stopping or decreasing output current modulation in order to project a dark image like a night view, electric discharge instability may occur so that a flicker sometimes appears in the projector image. Although this flicker usually disappears gradually in tens of seconds to several minutes by continuing the lighting under that lighting condition, it is very offensive to the eyes of the viewer of an image, so that the improvement thereof is demanded.